7. PROJECT SUMMARY Alzheimer's disease (AD) is one of the most significant medical and societal challenges of our time and yet no current intervention strategies can halt or modify the underlying disease course. By attaining a more thorough understanding of the cellular processes that underlie disease pathogenesis, and by linking molecular and biochemical changes to specific clinical manifestations of the disease, we believe that efficacious, mechanism- based therapeutic strategies can be developed. Neuropathologically, the AD brain is characterized by the accumulation of aggregates of the amyloid-? (A?) peptide and neurofibrillary tangles (NFTs) composed of the hyperphosphorylated microtubule-associated protein tau. Importantly, several studies have revealed an important link between A? and tau pathology. A? is derived from proteolysis of the ?-amyloid precursor protein (APP) following sequential cleavage by the ?- and ?-secretases. G protein-coupled receptors (GPCRs) are involved in key neurotransmitter systems that are disrupted in AD patients and are also associated with multiple stages of APP proteolysis, indicating an intimate association between GPCRs and the molecular pathways involved in AD. However, the true molecular nature of these relationships remains only partially understood. We have identified the orphan GPCR GPR3 as a key modulator of ?-secretase activity and determined that ?-arrestin 2 (?arr2), which belongs to a small family of multifunctional GPCR adaptor proteins, specifically interacts with the ?-secretase complex and, critically, is required for the GPR3-mediated effect on A? generation. These results support the hypothesis that ?arr2 may be a critical link between GPCR dysfunction and the ?-secretase complex in AD. We propose three multidisciplinary Aims to comprehensively investigate the mechanism through which GPR3 and ?arr2 cooperate to regulate ?-secretase function. Aim 1 will focus on determination of mechanism through which GPR3 and ?arr2 regulate of the activity, substrate selectivity, and heterogeneity of the ?-secretase complex. Aim 2 will focus on determination of the in vivo consequence of selective abrogation of ?arr2-dependent signaling on ?-secretase function, which is likely essential for triggering physiological and pathophysiological outcomes in mouse models of the disease. Aim 3 will focus on elucidation of the pathophysiological role that the GPCR kinases (GRKs) play in ?arr2 recruitment to GPR3, GPR3 phosphorylation, and/or modulation of ?-secretase function. The proposed studies will provide the first demonstration of the in vivo consequence of selective modulation of ?arr2-dependent signaling in AD pathogenesis and investigate the previously unappreciated role of GRKs in modulation of ?-secretase function. Results from these studies will not only address a major challenge in understanding disease mechanisms in AD, they will also provide new avenues for the development of potential therapeutic targets to mitigate and/or halt the neurodegenerative changes observed in this devastating neurodegenerative disorder.